Water Potential and Osmoregulation

Counting water molecules on each side of a membrane does not tell you where water is headed, because both pressure and dissolved solutes pull on water at the same time. Water potential, symbol psi, bundles both effects into one number measured in megapascals (MPa). Pure water at atmospheric pressure is set to 0 MPa, and adding solutes always drops water potential below zero, since dissolved particles tie up water molecules and lower their tendency to move.

Water potential equals pressure potential plus solute potential, written psi = psi_p + psi_s. Pressure potential comes from physical squeezing, like a plant cell wall pushing back on its contents, and is usually positive or zero. Solute potential comes from dissolved particles and is always negative or zero, calculated as psi_s = -iCRT, where i is the number of particles the solute splits into, C is molarity, R is the gas constant 0.0831 L MPa per mol K, and T is temperature in Kelvin. Once you have a single psi value for two regions, the rule is simple: water flows from the region with higher (less negative) psi to the region with lower (more negative) psi, no exceptions.

This single number connects directly to how cells regulate water across their membranes. A cell in a hypotonic bath has lower solute potential inside, so water rushes in, and some organisms counter this with structures like contractile vacuoles that pump excess water back out. In isotonic surroundings, water potential matches on both sides and net flow stops, even though molecules still cross both ways. Water potential is what turns the vague idea of osmosis into a number you can calculate and compare.